1. Field of the Invention
This invention relates generally to cooking devices, and more specifically, to large heating area deep fat fryer vats in which the cooking medium is heated by multiple circulating flows of heated gas from burner nozzles.
2. Description of the Prior Art
Recently, commercial users of deep fat fryer vats have become increasingly aware of the costs associated with operating and maintaining their cooking equipment. Unfortunately, conventional devices have significant disadvantages in the areas of thermal efficiency and maintenance.
Deep fat fryer vats currently in use are heated, among other known methods, by circulating heated combustion products from radiant burners through passages that are in thermal contact with the exterior surface of the vat. An example of such an exterior heating system is taught in U.S. Pat. No. 5,033,368, issued Jul. 23, 1991. The heat that builds up in the vat structure is then thermally transferred to the cooking medium. After the hot combustion products have contributed some of their heat to the system, they are removed to the atmosphere through some type of exhaust flue. This indirect approach to heating the actual cooking media results in various problems.
The thermal efficiency of the conventional indirect cooking method is less than ideal because the surface area through which heat is transferred is significantly less than the surface area surrounding the flow of the flue gases. Therefore, some of the heat will be dissipated without raising the temperature of the cooking medium.
Further, the heat that does raise the temperature of the cooking medium is not evenly applied to the exterior of the vat causing local "hot" or "cold" spots. Conventional devices have a single flow path for the heated combustion products. The temperature of the products decreases as they travel through the passage. Therefore, some regions of the vat will have a significantly higher temperature than other regions making it difficult to achieve a precise and uniform control of the cooking medium temperature. This problem with "hot" and "cold" spots is compounded when the medium is stationary i.e., it does not circulate through the vat.
A second barrier to controlling the cooking medium temperature that is encountered with conventional devices is the indirect heat transfer process. In order to heat the cooking medium, the vat structure must be at a higher temperature than the medium. Once the desired cooking medium temperature is reached, this remaining temperature differential can cause temperature "overshoot". If this occurs, the temperature of the medium can dangerously exceed the safe operating temperature.
Another problem resulting from the conventional technique is scorching of the cooking medium and any food debris that falls off (crumbs, etc.). Exterior vat heating can result in extremely high temperatures on the vat wall and slope areas, such that any resulting sticking and burning can damage the exterior vat walls. These high and nonuniform temperatures may also cause excessive thermal stresses in the vat walls and any welds therein. Any subsequent structural fatigue of the vat wall makes replacement necessary more often.
Finally, the conventional technique of welding the vat and heating apparatus together makes maintenance more expensive. The entire vat assembly must be replaced to repair or replace just a part, causing conventional devices to have high maintenance costs.
Interior passages for heated combustion products have been used in fryer vats. An example of such an interior heating system is taught in U.S. Pat. No. 4,751,915, issued Jan. 21, 1988. A passage through the lower extent of the vat carries heated combustion products from the front wall to the rear wall of the vessel. These straight heat exchanger tubes operate in a low pressure system with burner pressures of 0.25" water column to 0.50" water column.
Unfortunately, uniform temperature control is not achieved with these systems. The cooking oil in the lower region of the vat has a higher temperature because it is closer to the straight heat exchanger tubes. Previous interior heating systems also have only a single direction of flow for combustion products. Since combustion products cool as they transfer heat to the cooking medium, the straight heat exchanger tubes will transfer more heat to the front portion of the vat than to the rear portion. These factors combine to produce erratic and non-uniform temperature distributions.
Fryer vats are commonly equipped with lower unheated regions or cold zones in an attempt to collect food debris without scorching the vat surface. Straight heat exchanger tubes disrupt this process because they are located between the food and the cold zone. Debris settling towards the cold zone can burn or scorch on the outside surfaces of the heat exchanger tubes. This scorching damage results in greater maintenance costs.
These problems have not been overcome in the prior art. The prior art does not teach the capability of maintaining uniform cooking medium temperature, transferring heat efficiently, or preventing scorching damage to the surfaces of the fryer vat. Other drawbacks exist.